Here is my handy dandy chart. Round to the closest number.
Processor speed.
400 or less - Mame .55
433 - .56
466 - .57
500 - .58
533 - .59
566 - .60
600 - .61
650 - .62
700 - .63
750 - .64
800 - .65
850 - .66
900 - .67
950 - .68
1.0 G - .69
1.1 - .70
1.2 - .71
and add 100 more megahertz for each version after.
Those numbers are not carved in stone, but they do match up very nicely to what the computer will actually run full speed. When in doubt, round down. I generally check my chart, and then carefully read the whatsnew.txt to see what games/features/etc was added/changed in version indicated by the chart, and then I roll back versions until I hit a change/game I care about.